A known way of monitoring movement of structures such as buildings, bridges, dams, components/assemblies and the like is to use a surveying device such as a ‘total station’. A total station consists of a laser-based electronic distance measure (EDM) mounted on a mechanical platform. The base of this platform is typically placed on a stable, horizontal mount, such as a leveled tripod. The total station mechanics allow the EDM to be aimed in any direction by rotating around both the vertical axis and a horizontal axis. Encoders on each rotation axis allow the current direction of the EDM to be known.
A monitoring total station can include a motorised drive system that allows the EDM to be automatically aimed in any desired direction. The current direction of the EDM from the encoders together with the distance to the measurement point from the EDM enables the three dimensional location of a point to be determined.
Total stations are capable of measuring positions of points in three dimensions. However, they have one or more of the following limitations when tasked with monitoring a moving structure:                in order to monitor movement they require a reflective target to be attached to the measurement point;        to achieve the required resolution and accuracy they typically require several seconds to perform each measurement and are therefore not able to perform dynamic measurement, such as movement of a railway track as a train passes over it;        they can only measure a single point at a time; if multiple points are to be monitored then each must be measured sequentially i.e. data is only captured from a single point at a time;        measurement resolution can be limited by the angular resolution of the encoders (typically in the order of 1 arcsecond); and        some designs of reflective prisms can be a significant source of measurement error.        